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1. Field of the Invention
The present invention is directed to an x-ray device having a cold cathode. More particularly, the present invention relates to a housing for a miniature x-ray device having a cold cathode and method for fabrication.
2. Background Art
Doctors and scientists continually strive to find less invasive ways to treat patients. Using treatments that are less intrusive to a patient""s body, doctors can greatly reduce stress on the patient""s system. For example, laparoscopic techniques enable physicians to explore the interior of the body and perform surgery through a small opening in the skin. Less intrusive medical techniques are extremely beneficial when applied to cardiovascular diseases, reflux disease of the esophagus, and other diseases of vessels, cavities and lumens of the body. To that end, catheters and like devices have been developed to traverse the human cardiovascular or circulatory system. Such devices have the capability to enter small blood vessels with diameters of about two to four millimeters, and make hairpin turns along tortuous paths within the vasculature.
Many types of catheters have been developed to deliver medication, such as, Heparin, Dexamethasone, and the like, to treat cardiovascular diseases, such as, stenosis, restenosis and the like. Catheters have also been used to provide beta-irradiation along the wall of a blood vessel to treat illnesses such as, restenosis. Moreover, catheters have been applied to the esophagus to treat gastroesophageal reflux disease (GERD). Many other disorders could be treated with small, effective medical devices capable of accessing the interior of the body.
A typical x-ray catheter includes a coaxial cable and miniature x-ray emitter connected to the distal end. The x-ray emitter consists of an anode and cathode assembly mounted in a miniature vacuum tube. To activate the x-ray catheter, a high DC or pulse voltage (e.g., fifteen to thirty-five kilovolts (kV) range) is applied to the tube. At the cathode surface, a high electric field results in the electron field emission from the cathode. The electrons are emitted into a vacuum gap between the anode and the cathode, and are accelerated by the electric field. As the accelerated electrons strike the anode, X-ray radiation is produced from the electrons.
The proximal end of the coaxial cable is connected to a high voltage power source. Outside the patient""s body, the cable is secured to a pullback device, which moves the emitter along the blood vessel or another body cavity as it is being irradiated.
To be maneuverable within body cavities and blood vessels, it is desirable to use as small an x-ray device as possible. Miniaturization of x-ray emitters, however, imposes very stringent requirements on the dielectric strength and resistivity of the material that is required for the insulating shell. For coronary use, the wall thickness of the emitter should be as thin as 150-200 microns, and still be able to hold about thirty kV in voltage. This, in turn, requires the dielectric strength of the shell material to range between 150 to 200 kV/mm.
Very few dielectric materials can satisfy these requirements. For instance, ceramics, conventionally, have been used for insulation because they are relatively inexpensive, but they are ineffective for high voltage applications. Boron nitride is another conventional insulating material. However, boron nitride has poor vacuum integrity. Boron nitride is also a soft, mechanically weak material that is prone to cracking at the brazing joints. By using a long and sophisticated chemical vapor deposition (CVD) process, diamonds can be grown in special chambers to meet the resistivity and dielectric strength requirements. Although CVD diamonds have low thermal expansion properties, CVD diamonds have low absorption of x-ray and are not very accessible or machinable due to the CVD process.
What is needed, therefore, is an insulating material for the housing of an x-ray emitter that reduces cracking, and is easy and inexpensive to produce.
The present invention is directed to an insulating housing shell used to encase a miniature x-ray emitter. In a preferred embodiment, the housing shell is cut from a quartz monocrystal along its z-axis.
The x-ray emitter is suitable for inserting into a subject""s body and delivering x-ray radiation. The emitter includes a coaxial cable, having a proximal and a distal portion. The distal portion of the cable is coupled to the crystalline quartz housing shell. The housing shell provides a vacuum chamber enclosing an anode and a cathode disposed therein. The cathode has a granular surface and is operative with the anode and the cable to produce the x-ray radiation. The cathode is composed of a material that also allows it to act as a getter. The cathode can also include a diamond film on its surface that facilitates cold electron emission.
An advantage of the present invention is that crystalline quartz housing is compatible with existing brazing, soldering or other bonding technologies. This permits the vacuum chamber to be easily fabricated by known or future developed brazing, soldering or other bonding techniques.
Another advantage of the present invention is that the resistivity and dielectric strength properties of crystalline quartz do not substantially degrade or crack at higher temperatures.
Another advantage of the present invention is that crystalline quartz is easily accessible and machinable.